The present invention relates to a sliding structure for use in various kinds of machines, and specifically, relates to a sliding structure bearing a load applied to a rotating shaft. More specifically, the invention relates to a sliding structure including relatively slidable elements of an internal combustion engine, namely, a crankpin (or a connecting rod journal) and a crank journal (or a main bearing journal) of a crankshaft and bearing metals used as counterparts of the crankpin and the crank journal, respectively. Further, the invention belongs to a technical field relating to a microscopic surface structure of a sliding surface of the sliding element of the sliding structure.
The crank journal and the crankpin of the crankshaft are typically known as the sliding elements used in internal combustion engines. The crankshaft is coupled to a piston within an engine cylinder via a connecting rod and operated to convert the reciprocating motion of the piston into rotation. The crankshaft is rotatably supported by bearing metals at the crank journal and the crankpin. The bearing metals are generally made of Cuxe2x80x94Pb based alloy, Alxe2x80x94Sixe2x80x94Sn based alloy or the like.
Recently, for the purpose of performing efficient power transmission and serving for enhancing fuel economy and reducing emission in view of environmental conservation, reduction of friction loss is required in addition to improvement in output and revolution of internal combustion engines for automobiles. A journal of the conventional crankshaft is ground to have a surface roughness Ra of about 0.2-0.5 xcexcm, and then subjected to finishing with lapping tape to thereby have a surface roughness Ra of not more than 0.1 xcexcm. Surface roughness Ra is prescribed in JIS B 0601-1994. A thickness of a lubricating oil film formed between a sliding surface of the journal of the crankshaft and a sliding surface of a bearing metal is calculated at about 1-2 xcexcm or less under condition that the engine revolution number is in the usually operated range of 1500-2000 rpm. There has been studied reduction of sliding friction loss by decreasing surface roughness of the sliding surface of the journal of the crankshaft so as to smoothen the sliding surface. Japanese Patent Application First Publication No. 2000-504089 (corresponding to U.S. Pat. No. 6,095,690) and U.S. Pat. No. 4,538,929 disclose bearing metals having lubricating oil pockets on the sliding surfaces. These related arts aim to improve antiseizure properties of a sliding structure using the bearing metal and reduce friction loss thereof.
In order to smoothen the sliding surface of the journal of the crankshaft for the purpose of greatly reducing the friction loss, the sliding surface will be subjected to superfinishing. In this case, the production cost will increase. Otherwise, the sliding surface will be coated or impregnated with molybdenum disulfide superior in solid lubricating property. This will become expensive. Further, the layer of molybdenum disulfide formed on the sliding surface will be worn out during the sliding movement of the sliding surface. The durability of the journal of the crankshaft cannot be sufficiently obtained.
There is a demand to eliminate the above-described problems in the related arts. An object of the present invention is to provide a sliding structure in which opposed sliding elements are slidable relative to each other with a reduced resistance even if a lubricating oil film formed between sliding surfaces of the sliding elements has a relatively small thickness, the sliding structure serving for greatly reducing friction loss of the sliding elements.
In one aspect of the present invention, there is provided a sliding structure, comprising:
a first sliding element including a first sliding surface, the first sliding element being made of metal;
a second sliding element including a second sliding surface slidable relative to the first sliding surface via a lubricating oil film therebetween, the second sliding element being made of metal;
at least one of the first and second sliding surfaces including a microscopic surface structure including a base portion, dimples inward recessed from the base portion and separated from one another, a ratio of a sum of opening areas of the dimples to an area of the at least one of the first and second surfaces being in a range of 5% to 60%, and a peripheral portion defining the opening area of each of the dimples, the peripheral portion extending along a periphery of each of the dimples, the peripheral portion having a height smaller than a thickness of the lubricating oil film.
In a further aspect of the present invention, there is provided a sliding element slidable relative to a counterpart element via a lubricating oil film therebetween, the sliding element and the counterpart element being made of metal, the sliding element comprising a sliding surface having a microscopic surface structure including: a) a base portion; b) dimples inward recessed from the base portion and separated from each other, a ratio of a sum of opening areas of the dimples to an area of the at least one of the surfaces being in a range of 5% to 60%; and c) a peripheral portion defining the opening area of each of the dimples, the peripheral portion extending along a periphery of each of the dimples, the peripheral portion having a height smaller than a thickness of the lubricating oil film.
In a still further aspect of the present invention, there is provided a method for producing a microscopic surface structure in a first sliding element forming a sliding structure in cooperation with a second sliding element, the first and second sliding elements being made of metal and including sliding surfaces relatively slidable via a lubricating oil film therebetween, the microscopic surface structure including a base portion, dimples inward recessed from the base portion and separated from each other, a ratio of a sum of opening areas of the dimples to an area of the surface of the sliding element being in a range of 5% to 60%, and a peripheral portion defining the opening area of each of the dimples, the peripheral portion extending along a periphery of each of the dimples, the peripheral portion having a height smaller than a thickness of the lubricating oil film, the method comprising:
subjecting the sliding surface of the first sliding element to blasting to form a previous microscopic surface structure which includes the base portion, the dimples and projections extending outward from the base portion and along a periphery of each of the dimples, the projections having a second height larger than the height of the peripheral portion; and
subjecting the sliding surface of the sliding element to surface finishing to reduce the second height of the projections and form the previous microscopic surface structure into the microscopic surface structure.